Foam glass products have been known for a considerable time, as evidenced by the early patents to Haux, et al, U.S. Pat. No. 2,233,608; Ford, U.S. Pat. No. 596,669; U.S. Pat. No. 2,611,712; and Miller, et al, U.S. Pat. No. 2,233,631 wherein various chemicals and mechanical foaming processes are disclosed.
Recent interest in foam glass products is evidenced by the patents to Jones, et al, U.S. Pat. No. 3,459,565, wherein a foamable granular product is prepared by melting a glass in the presence of a gas under high pressure, thereafter cooling the glass so that upon reheating the glass will expand; to Marceau, U.S. Pat. No. 3,325,264, wherein glass is caused to foam by being heated to about 2000.degree. farenheit which causes dissolved magnesium oxide to decompose and gasify; to Slayter, U.S. Pat. No. 3,151,966, wherein foaming is caused by the dissolution of dissolved gas due to the crystallization of the glass and to Schott, U.S. Pat. No. 3,628,937, wherein glass is foamed by the action of a paddle which disperses gas throughout the glass as it is leaving a melting tank.
Although foam glass products produced by techniques of the prior art are useful for many purposes, foam glass products have not been useful as insulation at elevated temperatures. Most glasses have a substantial coefficient of thermal expansion and the existence of a temperature gradient of any magnitude across the glass causes cracking or crazing. Refractory glasses are generally those which withstand high temperatures and have a low coefficient of thermal expansion. Refractory glasses, however, require melting and processing at very elevated temperatures, thus consuming a considerable amount of energy to melt and causing rapid degradation of the refractory linings of the melting and processing tanks.
One technique of making a refractory foam product is disclosed in U.S. Pat. No. 3,592,619 issued to Elmer, et al. In the patent a high-silica glass foam is produced by melting a borosilicate glass composition, phase-separating the glass into a silica-rich phase and a borate-rich phase, leaching the borate-phase from the glass to produce a porous, silica-rich body having a pore size in the range of about 10 to 25 angstroms, impregnating the porous body with an aqueous boric acid solution, particulating the impregnated glass, drying the particulated glass to remove excess water and foaming the particulated silica at an elevated temperature of about 1300.degree. to about 1425.degree.C to form a foamed refractory body.
The Elmer process involves several critical steps, as indicated by the patent, the first being the leaching step which must produce a very fine network of pores to entrap therein the moisture that serves to expand the glass upon heating. Therefore, the glass must be carefully heated during phase-separation to produce a borate phase of the correct dimensions and heating must be avoided prior to leaching to prevent growth of the borate phase. The leached and impregnated glass must be flash fired so that the pores are closed very rapidly to prevent escape of water vapor. The glass is then heated to a temperature of about 1300.degree. to about 1425.degree.C to cause the glass to decrease sufficiently in viscosity so that it can expand. Foaming of a product at temperatures of 1300.degree.C and above requires processing equipment with a refractory lining. Since foaming occurs after leaching, the minimum density obtainable by Elmer's process is about 0.2 gm/cc, which is typical of glass foaming processes.
The process outlined in U.S. Pat. No. 3,592,619 is complicated and very time consuming in that a very large number of processing steps must be conducted to obtain a final product and the foaming of the final product must be done at very elevated temperatures in refractory lined equipment. Furthermore, because boric acid is introduced as a flux after leaching, the final product will not approach a pure silica content, as indicated by the thermal coefficient of expansion for Elmer's product of 8 .times. 10.sup.-.sup.7 per .degree.C as compared with 4 .times. 10.sup.-.sup.7 per .degree.C for pure silica. for pure silica.